1. Field of the Invention
Embodiments of the present invention generally relate to coherent light sources such as lasers. In particular, the invention is directed to a method and apparatus for providing intense and uniform illumination during short time intervals using a pulsed coherent light source.
2. Description of the Related Art
The integrated circuit (IC) market is continually demanding greater memory capacity, faster switching speeds, and smaller feature sizes. One of the major steps the industry has taken to address these demands is to change from batch processing silicon wafers in large furnaces to single wafer processing in a small chamber.
During such single wafer processing the wafer is typically heated to high temperatures so that various chemical and physical reactions can take place in multiple IC devices defined in the wafer. Of particular interest, favorable electrical performance of the IC devices requires implanted regions to be annealed. Annealing recreates a more crystalline structure from regions of the wafer that were previously made amorphous, and activates dopants by incorporating their atoms into the crystalline lattice of the substrate, or wafer. Thermal processes, such as annealing, require providing a relatively large amount of thermal energy to the wafer in a short amount of time, and thereafter rapidly cooling the wafer to terminate the thermal process. Examples of thermal processes currently in use include Rapid Thermal Processing (RTP) and impulse (spike) annealing.
A drawback of RTP processes is that they heat the entire wafer even though the IC devices typically reside only in the top few microns of the silicon wafer. This limits how fast one can heat up and cool down the wafer. Moreover, once the entire wafer is at an elevated temperature, heat can only dissipate into the surrounding space or structures. As a result, today's state of the art RTP systems struggle to achieve a 400° C./s ramp-up rate and a 150° C./s ramp-down rate. While RTP and spike annealing processes are widely used, current technology is not ideal, and tends to ramp the wafer temperature during thermal processing too slowly and thus expose the wafer to elevated temperatures for too long a period of time. These thermal budget type problems become more severe with increasing wafer sizes, increasing switching speeds, and/or decreasing feature sizes.
To resolve some of the problems raised in conventional RTP type processes various scanning laser anneal techniques have been used to anneal the surface(s) of the substrate. In general, these techniques deliver a constant energy flux to a small region on the surface of the substrate while the substrate is translated, or scanned, relative to the energy delivered to the small region. Due to the stringent uniformity requirements and the complexity of minimizing the overlap of scanned regions across the substrate surface these types of processes are not effective for thermal processing contact level devices formed on the surface of the substrate.
Pulsed laser annealing techniques have been used to anneal finite regions on the surface of the substrate to provide well defined annealed and/or re-melted regions on the surface of the substrate. In general, during a pulsed laser anneal process various regions on the surface of the substrate are exposed to a desired amount of energy delivered from the laser to cause the preferential heating of desired regions of the substrate. Pulsed laser annealing techniques have an advantage over conventional processes that sweep the laser energy across the surface of the substrate, since the need to tightly control the overlap between adjacently scanned regions to assure uniform annealing across the desired regions of the substrate is not an issue, since the overlap of the exposed regions of the substrate is typically limited to the unused space between die, or “kerf” lines.
Due to the shrinking semiconductor device sizes and stringent device processing characteristics the tolerance in the variation in the amount of energy delivered during each pulse to different devices formed on the substrate surface is very low. These device requirements have made the tolerance to variations in the delivered energy across the exposed surface of the substrate to be rather small (i.e., <5% variation). However, the use of a coherent light source, such as a laser, can introduce coherence effects, such as speckle and diffraction fringes, which can cause non-uniformities in the radiant energy which is incident on a small region of the substrate surface.
Various optical devices may be interposed between the laser source and the substrate to modify the beam for a particular laser annealing application. Such devices may include optical steering systems, pulse stretchers, beam spreaders, beam homogenizers, and other devices. As the coherent laser light passes through such devices, the light may scatter from rough surfaces or inhomogeneous media producing many coherent wavefronts which emit from the scattering sites and are subject to phase differences and/or intensity fluctuations. The coherent wavefronts may interfere to produce a random intensity pattern characterized by many small bright and dark points or spots, where the bright spots correspond to scattered waves that have interfered constructively and the dark spots to waves that have interfered destructively. Such an intensity pattern is also known as speckle and is a common phenomenon when coherent light is scattered from rough surfaces or inhomogeneous media. In addition to speckle, diffraction fringes may also be formed when the laser light passes by opaque objects or through apertures and lenses, for example.
To produce more uniform illumination or radiant flux density at the surface of the substrate during laser annealing, it is desirable to eliminate or minimize coherence effects such as speckle and diffraction fringes. One such method is to use rotating diffusers which have the effect of integrating multiple speckle patterns to produce a more uniform radiant flux at the surface of a target area. However, this approach is effective only if the rotation speed is significantly faster than the process integration time of interest. For example, if the process time is represented by a 50 nanosecond (ns) pulse width (also referred to as pulse duration) for a pulsed laser and a rotating diffuser has a rotation frequency on the kHz timescale, the rotating diffuser will be “strobed” by the laser and the substrate “will see” a speckle pattern. The use of a pulsed laser requires that coherence effects be removed well within the pulse width of the laser. In general, the use of mechanical motion of lenses and/or diffusers for eliminating coherence effects over nanosecond timescales is impractical.
Therefore, there is a need for a method and apparatus which reduces coherence effects, such as speckle and diffraction fringes, over short timescales and provides more uniform illumination at the substrate surface during laser annealing.